[llvm] [AtomicExpand] Implement SUB → ADD(-x) and FSUB → FADD(-x) (PR #221425)

[Akshay Deodhar via llvm-commits]

https://github.com/akshayrdeodhar approved this pull request.

LGTM with the nits for the test below- please rebase on the other two fixes before merging.

https://github.com/llvm/llvm-project/pull/221425